The present invention is directed to printed circuit boards, particularly multi-layer printed circuit boards. Specifically, the invention is directed to micro-etching of metal, particularly copper, printed circuitry by an oxidative process so as to improve adhesion of the metal circuitry to the dielectric layers of multi-layer circuit boards. Because the major portion of printed circuitry is copper circuitry, the invention will be discussed herein in primarily in relationship to copper printed circuitry. However, the invention is generally applicable to metal surfaces which can be oxidatively micro-etched to provide increased surface area; which correlates to increased surface roughness thus, discussions with respect to copper surfaces are to be understood to pertain to other metal surfaces which may be oxidatively micro-etched for surface area enhancement.
Successful fabrication of multi-layer printed circuit boards requires bonding together of copper and resin layers. However, direct bonding of copper and resin layers does not provide sufficient bonding strength. Therefore, it is common to improve copper-resin bonding strength by providing surface roughness to the copper surface, whereby mechanical bonding between the copper and resin is enhanced.
One common method of providing surface roughness is by depositing on the copper surface an oxide layer, such as cuprous oxide, cupric oxide, or the like. Formation of the oxide layer, which turns the pink copper surface a black-brown color, creates minute unevenness on the copper surface which provide an interlocking effect between the copper surface and resin, thus improving bonding strength.
However, copper oxides are readily degraded and dissolved upon contact with acid. Because various acid treatments are used in later stages of fabrication of multilayer circuit boards, oxide layer deposition has been problematic at best. In particular, through-holes are drilled through multilayer circuit boards, and the through-holes plated with copper to interconnect the circuitry of the several layers. As part of the copper-plating process, the through-holes are exposed to acids, such as sulfuric acid. Acid attack on the oxide layer in the region surrounding the through-holes is commonly referred to in the industry as xe2x80x9cpink ringxe2x80x9d, because as acid strips the black-brown oxide layer from the surface, a ring of bare pink copper becomes evident. The problem of copper oxide vulnerability to acid has been approached in a number of patents, e.g. U.S. Pat. Nos. 4,642,161 and 4,717,439.
An alternative to building up a copper oxide coating on the copper surface is to micro-etch the copper surface to roughen the same; and it is to this method of surface roughening that the present invention is directed. Most commonly, this etching solution is an aqueous solution of a mineral acid, such as sulfuric acid, and an oxidizer agent, such as hydrogen peroxide. Such a solution is described in U.S. Pat. No. 4,751,106, the teachings of which are incorporated herein by reference. The micro-etching process roughens the copper, probably by oxidation of the metallic copper; the resulting topography on the metal surface provides better adhesion to resin. The micro-etched surface appears pink/tan to dark brown, probably the result of the topography of the surface and some residual copper oxide that is formed in the etching process. Micro-etched copper circuitry, when exposed to acid-based copper plating of through holes, is likewise subject to formation of xe2x80x9cpink ringxe2x80x9d through acid attack of the copper circuitry traces around the through-holes. The formation of xe2x80x9cpink ringxe2x80x9d is detrimental at least to appearance and is a potential indication of a failure in multi-layer printed circuit boards.
It is a primary object of the present invention to provide enhanced surface topography of micro-etched metal surfaces, particularly deeper surface etching, so as to increase bond strength between the metal surfaces and polymeric materials used in processes of producing multilayer printed circuit boards. It is further an optional object of the present invention to provide acid-resistance to micro-etched copper circuitry so as to prevent or minimize acid attack problems such as xe2x80x9cpink ringxe2x80x9d. In this regard, it is to be understood that there are a wide variety of possible processing steps in forming printed circuitry, and the invention, though described relative to a few common processing procedures, is generally applicable to providing increased surface topography roughness and optionally acid-resistance.
In accordance with one aspect of the invention there is provided an oxidative aqueous etching solution for micro-etching metal surfaces so as to increase the surface area of the metal surface and optionally providing acid resistance to the micro-etched metal surface. The solution contains a proton source; an oxidizer agent; an azole compound; and a source of molybdenum ions. This solution is used to micro-etch a metal, e.g., copper surface. Very substantial improvements in surface topography, i.e., differential surface etching (peak to valley height difference), is achieved through inclusion of the molybdenum ion source in conjunction with the azole compound than with a standard peroxide sulfuric acid micro-etch technology. Additionally, optional inclusion of a thiazole and/or thiocarbamide compounds provides unusually high resistance to acid attack as evidenced by significantly longer times that it takes for an HCI solution to change the color of a micro-etched board from brown to pink. This improved acid resistance translates to elimination or minimization of xe2x80x9cpink ringxe2x80x9d in the formation of multi-layer printed circuit boards and elimination or minimization of other acid attack related problems in printed circuit board formation.
As an alternative to providing a thiazole and/or thiaocarbamide in the micro-etch solution, the micro-etched metal surface may be post-treated with an aqueous solution of a thiazole compound, a thiocarbamide compound or a mixture thereof.